There is evidence that patients with mental health conditions (MHC) may receive lower intensity and quality of care for several comorbid conditions, including diabetes mellitus. Results from our current "MEND" (Mental Health-Diabetes) study (VA HSR&D MR 200-41) indicate that among patients with diabetes mellitus, glycemic (blood sugar) and lipemic (cholesterol) control are worse for those with MHC than for those without MHC; it is not known why. The goal of the proposed study is to determine what modifiable provider-level and patient-level barriers contribute most importantly to this effect. To meet our analytic goals, we will draw upon data from existing sources: a national cohort of over 800,000 patients with diabetes receiving care from the Veterans Health Administration (VA) (DEPIC cohort), centralized national VA patient care data (including pharmacy prescribing records and lab results), Medicare claims data (to capture non-VA utilization), and, for a subset of our cohort, Survey of Health Experiences of Patients data from VA's Office of Quality and Performance (for information about patient-centered care, SF-12 scores to measure functional status, and use of non-VA services). Using this rich merged database, we will examine associations between MHC and various potential barriers to diabetes care, including provider-level barriers (failure to intensify pharmacotherapy in response to out-of-range lab tests, failure to monitor lab tests, failure to provide patient-centered care) and patient-level barriers (nonadherence to medications, risky utilization patterns, or failure to self-monitor blood sugars). Our findings will point to short-term opportunities for quality improvement, helping clinicians and administrators to focus their efforts for patients with MHC, a potentially vulnerable subpopulation. Over the longer term, they will inform the next generation of translational research, which will test interventions designed to optimize diabetes care for patients with mental illness.